1. Field of the Invention
The present invention relates to a whip antenna used with vehicles and more particularly to an attachment structure for the whip antenna.
2. Prior Art
Many different types of antennas are used with automobile radio receivers. Among them, single-length whip antennas can be manufactured inexpensively and performs well; that is they have good reception, a minimum requirement for antennas.
A single-length whip antenna normally includes an antenna element which is formed from a single, continuous rod-shape conductive material. The antenna element is designed to be matched more or less to a quarter wavelength of the FM band.
When the antenna is installed on a vehicle body, the antenna element remains outside of the vehicle body. Accordingly, in order to ensure that the antenna element will withstand load caused by obstacles during operation of the vehicle and or when the vehicle is washed, a material having high tensile strength and high recovery properties (such as stainless steel) is used.
Furthermore, the antenna is mounted to the vehicle body by securing the antenna element with screws to an attachment base which has been installed on the wall of the vehicle body. In this case, since the whip antenna element is formed from a high tensile material, workability (that is, machinability, etc.) is usually poor. As a result, it is difficult to cut thread thereon; thus, it is difficult to connect the antenna element to the attachment base with screws. Therefore, a special connecting means such as joint is usually interposed between the whip antenna element and the attachment base.
FIG. 2 shows a typical example of such a prior art arrangement. A joint (first joint) 2 is mounted on the base of a whip antenna element 1, and a male screw 3 projects from the bottom of the joint 2. Another joint (second joint) 6 is coaxially inserted in the top end of a cylindrical attachment base 4 via an insulator 5. A female screw 7 which engages with the male screw 3 is formed at the center of the second joint 6. A feeder terminal 8 is formed on the lower end of the second joint 6 so that the terminal 8 projects from the insulator 5 and is connected to a connector (not shown) on a feeder (not shown). A male screw 9 is formed on the outer surface of the upper portion of the attachment base 4 so that the base 4 can be screwed to the vehicle body.
FIG. 3 illustrates another example of the prior art. In this example, a female screw 10 is formed in the first joint 2 and a male screw 11 projects from the second joint 6. In all other aspects, the structure is exactly the same as that shown in FIG. 2, and the whip antenna element 1 and attachment base 4 are coupled together via the joints 2 and 6.
However, the above-described whip antennas have several disadvantages in that in order to insure workability, it is necessary that the joints be made of a material which has a lower tensile strength than the whip antenna element 1. However, the material must also be strong enough to withstand load.
Another greater problem is that even if a material is used for the joints which is adequate in terms of strength, the dimensions of the male screw (3 and 11) cannot be very large, since the screw coupling must be in a limited (relatively small) space. As a result, if the load which bends the whip antenna element 1 in the direction indicated by the arrows in FIGS. 2 and 3 is concentrated at the screws, the root portion of the male screws (3 and 11) may bend or break.
In view of the desirability of having as large space as possible during transport of the vehicles, it is desirable that whip antenna be detachable from the vehicle body. Except for this reason (to conserve space), there is no other reason for the antenna to be detachable from the vehicle body. Particularly, it is not absolutely necessary that the whip antenna element and attachment base be separated from each other and these components may be formed as an integral unit. In fact, such an arrangement has been employed.